Double-acting electromagnetic actuator

ABSTRACT

The invention relates to an electromagnetic actuator for a rapid linear motion with a limited length of stroke. The electromagnetic actuator includes a stationary arranged first coil ( 1, 6 ) and a second movable coil ( 2 ), with the winding of the stationary coil connected to controllable power source ( 7 ) and the winding of the movable coil is short-circuited without any galvanic connection to an external power source The ends of the winding of the movable coil of the electromagnetic actuator is short-circuited via a rectifier element ( 9 ), preferably a diode. The diode allows a current to be developed in one direction only in the winding of the second coil, said current being induced from an electromagnetic field generated by a current through the first coil. In this manner is a double-acting electromagnetic actuator with very low weight obtained, resulting in very rapid response and high reliability, due to the lack of any external electrical connections to the movable coil.

Present invention relates to an electromagnetic actuator having a rapidlinear motion with a moderate length of stroke.

STATE OF THE ART

It is previously known with coils being movable under the influence ofmagnetic fields. Such an example could be found in loud speakers withstationary permanent magnets having a movable voice coil arranged in themagnetic field induced by the permanent magnet. The winding of the voicecoil is connected to an external power source, and by current controlthe coil could be given the intended motion. A drawback with thissolution is that the external connections are movable and subject to apotential interruption.

In U.S. Pat. No. 5294850 is further known a device wherein anelectromagnetic field-effect could launch missiles. In this solution astationary coil is used, which affects a coil arranged on or in contactwith the missile to be launched. The movable coil is lacking anyexternal connections and the winding is short-circuited, oralternatively divided into several coil segments, and wherein theelectromagnetic field is controlled by the current in the stationarycoil.

In U.S. Pat. No. 1066081 is shown in an alternative embodiment, i.e.FIGS. 4 and 5, a relay having a stationary and a movable coil. Thewinding of the movable coil is connected to a stationary circuitbreaker, and the movable coil is affected in a controlled manner in onedirection by said circuit breaker and the magnetic field induced fromthe stationary coil.

A conventional solution, in order to obtain a double-acting actuator,utilize a stationary solenoid and a movable iron core, which iron coreis forced towards a first end position using a return spring.

When the iron core is activated towards the second end position, thenthe force from the electromagnetic field must overcome the counteractingforce from the return spring and initiate movement of the mass of theiron core. This will bring about a decrease in response, due to therather large mass of the actuator and need to overcome the force fromthe return spring.

OBJECT OF THE INVENTION

The object of the invention is to obtain an electromagnetic actuatoruseful for most situations where a double acting and rapid movement witha moderate length of stroke is requested. Another object is to obtain anelectromagnetic actuator with a quick response, yet another object is toobtain an electromagnetic actuator lacking any electrical connections tothe movable part of the electromagnetic actuator.

An object with a further refined embodiment is to be able to obtain afeed-back signal of the position of the actuator, whereby an improvedcontrol with increased accuracy of movement of the actuator could beobtained.

SHORT DESCRIPTION OF THE INVENTION.

The inventive electromagnetic actuator is an electromagnetic actuatorfor a rapid linear motion with a limited length of stroke, with astationary arranged first coil and a second movable coil, wherein thewinding of the stationary coil is connected to a controllable powersource. The winding of the movable coil is short-circuited without anygalvanic contact with external power sources in that the ends of thewinding of the movable coil are short-circuited via a rectifier element,which rectifier element only allows current to be developed in onedirection in the winding of the movable coil, which current in themovable coil is induced from an electromagnetic field generated by acurrent through the stationary coil

The inventive electromagnetic actuator is a double actingelectromagnetic actuator that can be obtained with less dead weight ofall moving parts and which will give a rapid response of the actuator.The electromagnetic actuator will also exhibit a lack of any electricalconnections to the movable part, which will give a high order ofreliability.

Other distinguishing features and advantages of the invention will beevident from the characterising parts of the claims, and followingdescription of embodiments. The description of embodiments are made byreference to figures from the following list of figures.

LIST OF FIGURES

FIG. 1. shows in a side view an inventive electromagnetic actuator,

FIG. 2 shows the electromagnetic actuator in FIG. 1 as seen from above,

FIG. 3a, 3 b and 3 c shows respectively the current through thestationary coil, the current through the movable coil and the forceinduced by the movable coil,

FIG. 4. shows an analog circuit for detection of the position of themovable coil,

FIG. 5, shows an alternative solution for the actuator.

DESCRIPTION OF EMBODIMENTS

In FIG. 1 is shown the inventive electromagentic actuator. A stationarycoil 1,6 is wound upon a core 5, which preferably is a ferrite-core. Inthis embodiment the stationary coil is divided into two coil segmentsconnected serially, each wound around one leg of the core having twolegs in parallel. In an alternative embodiment the core could bemanufactured by laminated sheet metal. But a ferrite-core, even thoughmore expensive, is preferred.

A controllable power source 7 is connected to the stationary coil,controlling the current I_(p) through the stationary coil.

A coil 2 movable in relation to the stationary coil, is wound on a coilformer 3. The coil former is preferably guided by a third leg of thecore 5, which third leg is in parallel with the legs upon which thestationary coil is wound, and said third leg located between these twolegs.

The coil former and the coil wound thereupon is located in an air gap 4between the two legs of the stationary coil.

In order to retain the movable coil on the coil guiding leg of the core,is the coil former 3 is equipped with a flange 10 at the lower partthereof as shown in FIG. 1. The upper and lower surface of the flange 10acts as a first and second stop lug, each interacting with a first andsecond stop lug respectively of the core. The first stop lug 11 of thecore is formed by two radially and inwardly directed protrusions of thecore legs, upon which the core segments are wound. The first stop lugs11 limiting the movement of the movable coil in a first protrudedposition.

The second stop lug 12 of the core is limiting the movement of themovable coil in a second retracted end-position of the movable coil 2 inrelation to the stationary coil 1,6.

In the embodiment shown the coil former cylindrical, apparent in FIG. 2,is with an integrated actuator arm 8. The coil former couldalternatively also be given other shapes, for example with a rectangularor polygonal cross sections without departing from the invention.

The coil wound at the movable coil former is short-circuited via a diode9, which diode only conducts current in one direction. This diode couldbe substituted with any equivalent type of component, which componentonly will conduct current in one direction in the second movable coil,which current is induced from an electromagnetic field generated bycurrent through the first and stationary coil.

The function of the electromagnetic actuator is described in detail withreference to the current-and force-graphs shown in FIG. 3a-3 c as afunction of time. These principle graphs have been obtained afterpractical tests of an embodiment corresponding to the embodiment shownin FIG. 1. In FIG. 3a is shown the current I_(P) through the stationarycoil 1,6, which current is controlled in an conventional manner via theconnected power source 7.

In FIG. 3b is shown the current through the movable coil 3, whichcurrent is induced by the electromagnetic field generated by thestationary coil. In FIG. 3c is shown the force F obtained at theactuator-arm 8, when the movable coil 2 is influenced by the magneticfield in the air-gap 4.

In the embodiment shown, a first “pull cycle” is defined, correspondingto a movement of the movable coil inwards, i.e. in a downward directionin FIG. 1. At start of the pull-cycle the current I_(P) is initiated inthe stationary coil 5,6, which generates a magnetic field that in turnwill induce a current I_(d) in the movable coil 4. The current in thestationary coil reaches its maximum value at the point of time A, atwhich time also the current in the movable coil and the force obtainedfrom the actuator an 8 reaches maximum values respectively. Shortleyafter the point of time A reduction of the current I_(P) is initiatedthrough the stationary coil. The reduction will result in that also thecurrent through the movable coil will decrease. The force F developedwill follow the equation;

F=B·I_(d)·L,

where B is the strength of the magnetic field and L the length of theconductor located in the magnetic field, and where a force is developedduring the entire cycle.

In order to maintain a continuous application of a force towards theretracted position, this sequence is repeated continuously. In thefigure, however, only two sequences during the pull cycle are shown.

In the “push-cycle”, corresponding to a movement of the movable coiloutwards, i.e. in a upward direction in FIG. 1, a current is initiatedin the stationary coil in the reversed direction. This current willgenerate a magnetic field having an opposite direction in relation tothe pull-cycle, and which magnetic field is likely to induce a currentin the movable coil when the field and current declines. Immediatelyafter the point of time B, the current through the stationary coil issubject to decrease, whereby the magnetic field starts to induce acurrent through the movable coil in the same direction as the currentinduced during the pullkycle. A force F, following the same forceequation as mentioned earlier (F=B·I_(d)·L), is obtained, and directedin the opposite direction in relation to the pull-cycle, due to thechange of spin of B. In the figure, however, only two sequences duringthe push-cycle are shown.

Tests have also proven that a determination of the position of themovable coil could be made by detection of θ, see FIG. 3, whichcorresponds to dI_(p)dt, i.e. the first order derivative value of thecurrent through the stationary winding. The parameter θ decreases withdecreasing exposure of the movable coil in the magnetic field. Thisdetermnination of position could preferably be performed by means ofconventional analog circuitry.

In FIG. 4 is shown in principle such basic analog circuitry. In thisembodiment is used a simple operational amplifier OP, which is connectedthrough the resistance R and the capacitor C, such that the input signalI_(p) produce the output signal dI_(p)/dt. In practical implementationswill the circuitry will require some supplementing logic in order toobtain a correct analysis and sampling of the signal.

The inventive electromagnetic actuator could also in a further improvedembodiment be controlled as of position, where the processed signal ofposition is used as a feed-back signal of the position. By modulation ofthe pulse-width during the pull- and the push-cycle, the actuator couldbe imparted any arbitrary position between the two end positions.

In order to ensure that a predetermined lowest order of force shall beobtained from the actuator, the current lp through the primary coilcould be controlled at a higher level in terms of absolute value. i.e.at a level where I_(p) is not allowed to be reduced to a zero-level.This could contribute to an improved efficiency.

The invention could within the scope of the claims be modified in anumber of ways. As an example the core could be given another shape andthe stationary coil could have only one coil segment. In FIG. 5 is anexample of an embodiment adapted for production, where the primarywinding 6′ is wound upon the center leg of the coil 5′, concentric withthe secondary winding 2′. This embodiment will give an improvedtransformer coupling, where the core could be given a form axiallysymmetrical in relation to axis X. At the same time the primary winding6′ is given a improved protective enclosure.

In case of an implementation in power demanding applications, therectifier element could be replaced by MOSFET technology, in order toreduce any power losses through the rectifier element. By implementationof MOSFET technology the potential drop in the conducting directioncould be reduced from an order of 0.7 volts to only a fraction thereof.

What is claimed is:
 1. Electromagnetic actuator for a rapid linearmotion with a limited length of stroke, with a stationary arranged firstcoil and a second movable coil, wherein the winding of the stationarycoil is connected to controllable power source and the winding of themovable coil is short-circuited without any galvanic contact withexternal power sources characterized in that the ends of the winding ofthe movable coil are short-circuited via a rectifier element whichrectifier element only allows current to be developed in one directionin the winding of the movable coil, which current in the movable coil isinduced from an electromagnetic field generated by a current through thestationary coil.
 2. Electromagnetic actuator according claim 1characterized in that the rectifier element is a diode. 3.Electromagnetic actuator according claim 1 characterized in that thefirst coil is wound upon a core, and wherein the second movable coil isarranged upon a coil former which in turn is arranged with an air gap toand guided by a protrusion of the core.
 4. Electromagnetic actuatoraccording claim 2 characterized in that the rectifier element whichrectifies the current of the movable coil is arranged integrated withthe coil former and the winding of the movable coil.
 5. Electromagneticactuator according claim 4 characterized in that the coil former uponwhich the second movable coil is firmly arranged, also includes anintegrated actuator arm.
 6. Electromagnetic actuator according claim 5characterized in that the coil former includes a first and second stoplug which in cooperation with a first and second stop lug respectivelyupon the core, will limit the movement of the movable coil between afirst and second end position in relation to the first stationary coil.7. Electromagnetic actuator according any of the proceeding claimscharacterized in that the winding of the first coil of the actuator isconnected to detection means by which a detection of a valuecorresponding to the speed of change of the current through the windingcould be made.